1. Field of the Invention
This invention relates in general to fluid dynamic flow technology. More particularly the invention relates to a method and apparatus for suppressing vibrations caused by pressure oscillations in structures having cavities located therein that are moving through a media.
2. Description of the Related Art
In the testing and development of systems for reducing and/or eliminating vibrations found to occur in missiles during flight due to pressure oscillations, i.e. noise, in the cavities once the retractable wings are deployed, examination of oscillations measured in experimental cavities indicate that such oscillations may be suppressed by the installation, at the leading edge of the cavity, of members that disrupt the formation of coherent flow structures. The destruction of the coherent flow structures eliminates the source for noise generation related to impingement of these coherent flow structures on the trailing edge of the cavity. These same coherent flow structures are evident in outer space vehicles, ground surface vehicles and water vehicles, both surface and subsurface craft, having cavities exposed to the media through which the vehicle moves.
Devices have been known in the past for influencing the flow of a medium to control its effect on a body of a vehicle, such as an air foil or other portion of the structure of an aircraft or missiles moving relative to the medium. Vortex generators thus have been implemented in numerous aircraft systems to generate fluid elements upstream of locations where boundary layer separation is incipient. By selective deployment of vortex generators, boundary layer flow separation may be prevented and strong variations in the flow avoided.
One apparatus for influencing the boundary layer on the surface of a body moving through a medium is disclosed in U.S. Pat. No. 4,836,473. As disclosed in said patent, boundary layer influencing members are rigidly secured to the skin of an aircraft, including the wings, the tail and empennage, and the body adjacent to the tail. The members are characterized as vortex generators and boundary layer diverters and are located on the skin of the aircraft directly in the path of the airstream. Where flush air intakes are to be configured, the members are placed upstream of the entrances to the intakes themselves.
Another arrangement for modifying the aerodynamic characteristics of an object in flight is shown in U.S. Pat. No. 4,650,139. As disclosed in the patent, and as applied to a space vehicle system, an attempt is made to alter the aerodynamic flow around the vehicle by attaching to the external tank and orbiter of a space shuttle, a spike or rod to reduce aerodynamic heating and drag on the vehicle during its ascent to orbit.
One other construction employed to reduce the drag between a surface of an object and a free stream moving relative to it is shown in U.S. Pat. No. 4,693,201. As the patent discloses, the construction utilizes a plurality of longitudinally extending, parallel, and spaced apart linear vortices which extend transversely of the free stream. Although the vortices retain the same relationship with each other, they traverse the surface in the same direction as the free stream but at approximately half of its speed. A plurality of apertures are utilized in the surface construction.
The device as used in U.S. Pat. No. 4,706,910 causes break-up of large scale disturbances in the boundary layer of the flow field. Longitudinal striations forming vee-shaped grooves, and parallel ribbons suspended transversely across the flow surface, reduce skin friction on aerodynamic and hydrodynamic surfaces.
In an apparatus for reducing drag, U.S. Pat. No. 4,736,912 discloses a body having relative movement with a fluid and a surface zone having therein a plurality of longitudinal channels. Each channel includes a longitudinal gap which extends in a direction parallel to the direction of the flow. A reduction in turbulent drag is experienced in the body during movement.
Although generally satisfactory results can be considered to develop from application of the above-exemplary related art systems, their main purpose is centered on the creation of vortices for achieving the various results as explained in the preceding paragraphs hereinabove. The present invention is directed to the elimination of acoustic oscillation by preventing the generation and growth of vortices in a simple and economically feasible modification of existing vehicles or crafts which are susceptible to design consideration induced oscillations.